A feeble eclipse will slightly darken the Moon before dawn tomorrow across much of the United States. The eclipse will be underway as the Moon sets around the crack of dawn. The eclipse is so subtle, though, that it’s tough to see.

Lunar eclipses can take place only at full Moon, when the Moon lines up opposite the Sun in our sky. When the alignment of Sun, Earth, and Moon is just right, the Moon passes through Earth’s shadow. The Moon’s orbit is tilted a bit with respect to the Sun, though, so most months the Moon misses the shadow entirely.

And even when it does pass through the shadow, the eclipse isn’t always a good one.

That’s because Earth’s shadow has two parts — a dark inner part, which cloaks the lunar surface in darkness, and a hazy outer part, in which a lot of sunlight filters through. And it’s that part of the shadow — the penumbra — that’ll cover the Moon early tomorrow.

At the peak of the eclipse, a little after 8:30 Central Time, the penumbra will cover more than 90 percent of the Moon’s diameter. If the Moon is still up at your location then, you may notice that one edge of the Moon appears a little darker than the opposite edge. The darker edge is the one that’s closest to Earth’s inner shadow. Except for the time around the peak, though, that shadow will do little to dim the brilliance of the full Moon — which is known, by the way, as the Frost Moon. We’ll have more about that tomorrow.